(1) Field of the Invention
The present invention relates to a driving method for solid-state imaging device and a solid-state imaging device that improve fidelity of imaging and a signal processing time through noise reduction caused by a dark signal generated in a peripheral circuitry.
(2) Description of the Related Art
CCDs and MOS image sensors are generally used as solid-state image sensors in digital cameras, video cameras, and the like. The solid-state image sensors have been improved in recent years, and high-resolution solid-state image sensors have come to allow high-definition images to be imaged.
That dark currents caused by crystal defects in pixels are a factor for reducing the quality of the imaged images is known as the characteristics of the aforementioned solid-state image sensors.
Although dark outputs of the solid-state image sensors have been reduced, when imaging through long exposure exceeding several seconds to several tens of seconds, the dark outputs of the solid-state image sensors not only increase noise but also negatively influence color reproduction through signal processing. As a result, a problem emerges that the fidelity of the imaged images is decreased. The following conventional techniques have been disclosed as compensation methods for such image deterioration.
In Japanese Unexamined Patent Application Publication No. 2005-62993 (Patent Reference 1), since dark currents increase and decrease with a sensor temperature and a signal accumulation time, an image including only dark output components is obtained by performing a storage operation for a period of time with the whole solid-state image sensors shielded from light, immediately before or after actual imaging, and the dark output components are subtracted from an actual imaging signal on a pixel basis, the period of time being equal to the time spent for the actual imaging. This can reduce deterioration in image quality due to fixed pattern noise caused by dark outputs including a portion of change in the temperature and a portion of the accumulation time (compensation through actual dark subtraction).
Although this method is a very effective compensation method for image deterioration caused by the dark outputs, as the generation of the dark outputs temporarily fluctuates in principle, it is impossible to separately obtain dark output components that are completely the same as the dark output components at the time of imaging. Thus, compensating the dark outputs from image information inevitably causes compensation errors. Therefore, even when performing the compensation is assumed, it is desirable to suppress the generation of the dark outputs at the time of imaging as much as possible. In order to reduce the compensation errors as much as possible, it is necessary to perform the compensation only in the case where a size of the dark output components is larger than a given size and to reduce the temporal fluctuation by spending more than a given period of time in obtaining the dark output components. Furthermore, when conditions such as a temperature of the solid-state image sensor are different from those at the time of imaging, there is a chance that dark output components different from those at the time of imaging are obtained. In fact, although performing the accumulation operation for the period of time equal to the time spent for the actual imaging right after the actual imaging is common as a method of obtaining dark output components of an image, this has been an obstacle to camera operability because an effective imaging time becomes longer.
In Japanese Unexamined Patent Application Publication No. 2007-28338 (Patent Reference 2), dark outputs of a solid-state image sensor that result from a peripheral circuitry included in the solid-state image sensor are to be solved. Since, in comparison with thermal dark outputs generated from the pixels of a solid-state image sensor, the dark outputs of the solid-state image sensor due to the peripheral circuitry have the large maximum level in the solid-state image sensor, the dark outputs of the solid-state image sensor significantly deteriorate a dynamic range in the solid-state image sensor in which an amount of signal saturation is limited. Patent Reference 2 discloses a technique of preventing, through prohibition of imaging, or alerting a person imaging to, the deterioration in image quality such as the deterioration of the dynamic range from occurring by compensating the fixed pattern noise due to the dark outputs, by detecting nonuniformity in an imaging region of the dark outputs in the case where locally strong dark outputs are generated in the proximity of the peripheral circuitry in the imaging region of the solid-state image sensor.